Whether opioids affect tumor growth has been an important but unanswered question in tumor biology for decades. While opioids are widely used to treat pain in cancer patients, our preliminary data show that morphine and other opioids in clinically relevant doses can promote angiogenesis to enhance breast tumor growth and metastases in mice by stimulating pro-angiogenic and survival promoting signaling. Conversely, the opioid receptor antagonist naloxone inhibits tumor growth, further corroborating that opioid receptors play a critical role in tumor growth. We hypothesize that (a) specific opioid receptors regulate tumor angiogenesis through distinct intracellular signaling pathways, and (b) endogenous and clinically relevant levels of opioids promote tumor angiogenesis and growth, and naloxone inhibits this process. We will test these hypothesis with: Aim#1. Test that tumor growth, tumor angiogenesis, metastasis and survival are: [a] promoted by opioids in a dose and time dependent fashion [b] prevented and/or inhibited by naloxone in a dose and time dependent manner [c] modulated by endogenous opioids. Aim#2. Identify the specific opioid receptor(s) involved in the modulation of tumor angiogenesis, tumor growth and metastases using [a] receptor specific agonists and antagonists and [b] opioid receptor knockout mice. Aim#3. Examine if opioids modulate blood flow and vascular permeability in tumors. Aim#4. Identify signaling mechanism(s) induced by specific opioid receptors and their inhibition to prevent tumor growth. Aim#5. Prevent/delay tumor growth by naloxone and its more potent bivalent derivatives. Human and mouse breast tumor models including C3-TAG transgenic, MCF7, SCK, T241 fibrosarcoma (non-breast) in nude or immunocompetent A/J or C57 background wild type mice will be used for extensive in vivo experiments. Human dermal microvascular endothelial cells, SCK and C3-TAG tumor cells and human embryonic kidney cells transfected with different opioid receptors will be used for in vitro studies. If our hypothesis are true, this Proposal may lead to the development of novel therapeutics for breast cancer treatment and to altered clinical decision making in treating pain in cancer patients.